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During a climb at a constant Indicated Air Speed IAS, the True Air Speed TAS
  • A
    increases when switching to climb at a constant Mach number.
  • B
    decreases due to the decreasing air density.
  • C
    remains constant after the tropopause.
  • D
    increases, and the aircraft might exceed MMO.

Effect on Mach Number of Climbing at a Constant IAS

Temperature decreases with increasing altitude, local speed of sound will decrease as altitude is increases. It is known that if altitude is increased at constant IAS the TAS will increase.
Mach no. = TAS / LSS

Climbing at constant IAS, TAS will increase and therefore the Mach no. as well. But the Mach no. will increase at a bigger rate than TAS, because the LSS will decrease with altitude. Beyond a certain altitude, the Mach number gets too high and MMO can be exceeded. Therefore, at some lower altitude the aircraft needs to change its climb profile to a constant Mach number climb.

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